The present invention relates generally to transmitters for AM stereo radio broadcasting systems and, in particular, to techniques for reducing distortion by modifying the stereo signal encoding and multiplex modulation portion of such transmitters.
In AM stereo radio broadcasting systems a signal component representative of the sum of left (L) and right (R) input stereo audio signals (L.sub.T +R.sub.T) is amplitude modulated on a carrier. A second signal component, representative of the difference between the L and R signals (L.sub.T -R.sub.T) is multiplex modulated on the same carrier, using phase or frequency modulation techniques. The (L.sub.T +R.sub.T) component is equivalent to monophonic information, whereas the (L.sub.T -R.sub.T) component conveys the stereophonic information. When both components are recovered in an AM stereo receiver they may be combined in such a way as to develop two output audio signals L.sub.R and R.sub.R which are representative of the original L and R stereo input signals that were supplied to the transmitter.
To insure accurate stereo reproduction in AM stereo receivers it is, of course, desirable to have L.sub.T =L.sub.R and R.sub.T =R.sub.R. However, distortion may result from various causes in the transmitter (including its antenna system), during propagation from the transmitter to the receiver, and in the receiver itself. It is desirable, therefore, to reduce such distortion in order to improve the accuracy of stereo reproduction at the receiver.
Although prior art techniques have been effective in reducing distortion introduced in AM stereo transmitters to levels which have been found to be acceptable in actual listening tests, nevertheless, it would be desirable if distortion in AM stereo transmission and reception could be reduced even further.
For example, with respect to the basic independent sideband (ISB) AM stereo system disclosed in the inventor's U.S. Pat. No. 3,218,393, the improvement disclosed in the inventor's U.S. Pat. No. 3,908,090 reduces certain distortion which was present in the basic system.